Thursday, October 15, 2015

Multi-turn potentiometer modification for cheap power supplies

As covered in an earlier post, I have four MC Power/MC Voice NG1620-BL power supplies. These serve me quite well, although adjusting the current limit at the low end of the scale is quite fiddly. Some time ago (a year or so) I replaced the potentiometers on one unit with multi-turn wire wound potentiometers. This simple modification completely fixed all problems with setting the output. The first time around I only ordered two potentiometers as they are quite expensive. Now I have replacement pots also for the rest of them.

Figure 1. Chinese multi-turn wire wound potentiometers.

I ordered the potentiometers off eBay. They are nominally 6.8k wire wound 10 turn potentiometers (WXD3-13-2W). Made in China, perhaps of brand Xinghuo. Based on how they look, I suspect they are recycled from old hardware. Anyway, these are the same ones I used for the first unit and I have been happy with them.

Figure 2. De-solder these.

Figure 3. Pots came off easily.

Figure 4. Close up on the old pots.

The first thing of course was to remove the original pots. The knobs just pull off, and the pots themselves are secured to the case with hex nuts. I don't have a long enough socket, so I just used pliers to loosen the nuts. An important thing to check is which way the pots are wired. Both pots in this power supply are connected in the way, in which the counter-clockwise end gives the minimal resistance. For the new pots this means that I need to connect to the terminal closest to the shaft in addition to the wiper, which is the terminal furthest away from the shaft.

Figure 5. The new pots are slightly too long to fit between the board and the front panel.

Figure 6. Board mounts unscrewed to make space to solder new pots in.

Figure 7. New holes drilled to mount the board slightly further back. New pots fit nicely.

The new pots use the same thread, so I just re-used the washers and hex nuts from the original ones. However, the new pots are slightly too long to fit nicely between the front panel and the main board. However, there is more than enough space in the enclose, so I just drilled new mounting holes for the board and moved it slightly back.

Figure 8. 0.8mm welding wire bent into U-shapes.

Figure 9. This is how the U-shaped wires attach to the pot. The knob goes over this with a drop of hot glue.

After closing everything up I still needed to put the knobs on. The shafts are different, so the old ones don't go straight on. I cut short lengths of 0.8 mm welding wire and bent them into U-shapes. This grabs the slot on the end of the shaft and slides in the knob quite tightly. A small drop of hot glue secures them in place. This sounds like a kludge, but it has held up perfectly in the one unit I modified earlier.

Figure 10. Voltage is now easy to adjust within 10 millivolts. Current is easy to adjust within 1 milliampere.

Figure 11. I have four of these power supplies. Two on the left are still unmodified, while the two on the right now have this modification.

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